1. Field of the Invention
The present invention relates to a process for producing a polyurethane lens The polyurethane lens produced according to the process of the present invention is used as various optical lenses such as spectacle lens, camera lens and the like.
2. Description of Prior Art
In recent years, demand for plastic lenses as spectacle lens has increased both in Japan and other countries. These plastic lenses are generally made of a polymer obtained by subjecting diethyelne glycol bisallylcarbonate (hereinafter referred to as DAC) to cast polymerization, or in some cases of a polymethyl methacrylate, a polystyrene, a polycarbonate or the like subjected to injection molding.
DAC resin made lenses have merits of being lighter, less breakable and more easily tinted than glass-made lenses and can meet a recent fashionable need of combining a large-sized frame and a color lens.
However, the DAC resin has a refractive index (hereinafter referred to as N.sub.D) of 1.500 which is smaller than the N.sub.D of glass (1.523, making large the thickness of the DAC resin-made lens. Accordingly, the DAC resin lenses are not much liked by users requiring a high diopter lens.
A number of proposals have been made in order to improve the above-mentioned problems of the DAC resin lenses. For example, Japanese patent application Kokai (Laid-Open) No. 217301/1985 proposes a copolymer of a polyisocyanate, a polyol and an aromatic vinyl compound, This copolymer has a high N.sub.D of at least 1.60, making it possible to produce a lens of small thickness. However, the copolymer has an optical dispersion (hereinafter referred to as .nu..sub.D of about 30 and gives striking color dispersion Therefore, it has very limited applications as a spectacle lens. Further, in the production of the above copolymer, it is difficult to control the reaction between the polyisocyanate and the polyol; therefore, it is necessary to reduce the content of the polyisocyanate component and the polyol component in the monomer mixture. This inevitably leads to the production of only a copolymer low in number of urethane bond, and the polymer will not have good impact resistance inherently possessed by urethane bond.
Japanese patent application Kokai (Laid-Open) No. 164501/1984 proposes a copolymer of halogen-containing aromatic vinyl compounds. While this copolymer has a very high N.sub.D of 1.60-1.64 and is effective in producing a lens of small thickness, the copolymer has a .nu..sub.D as low as about 30 and has limited applications as a spectacle lens. In addition, the copolymer is said to have poor impact resistance and low tintability.
Japanese patent application Kokai (Laid-Open) No. 217229/1985 and Japanese patent application Kokai (Laid-Open) No. 199016/1985 propose a copolymer of a polyisocyanate and a sulfur atom-containing polyol and a copolymer of a polyisocyanate and a polythiol, respectively. Since these polyurethanes have a high N.sub.D of 1.56-1.64 and a small specific gravity of 1.22-1.44, they are suitable as materials for thin and light spectacle lenses, in particular. Further, these polyurethane lenses inherently have excellent impact resistance and tintability. When the above polyurethane lenses are produced according to cast polymerization, the glass-made or metallic mold used is coated with a silicon- or fluorine-based releasing agent in order to endow the produced lens with good releasability. This causes the remaining of the releasing agent on the surface of the lens released from the mold, which invites the color unevenness of the tinted lens and the peeling of the coating film formed on the lens. Moreover, the use of the releasing agent requires a long-time washing treatment for used mold.
In order to solve the above problems of the process comprising coating a mold with a releasing agent, it was proposed to add a releasing agent to a monomer mixture instead of coating a releasing agent on a mold and then subject the whole mixture to cast polymerization. In this case of adding a silicon- or fluorine-based releasing agent to a monomer mixture comprising a polyisocyanate and a polythiol, however, the amount of the releasing agent being in contact with the inner surface of the mold is very small as compared with a case of coating the releasing agent on the mold and, as a result, it is difficult to release the produced polyurethane lens from the mold smoothly The releasability of the lens from the mold can be improved by increasing the amount of the releasing agent added to the monomer mixture, but the use of the releasing agent in a large amount causes the cloudiness of the lens produced or the separation of the polymer from the mold during polymerization, thus making it impossible to obtain a lens of high commercial value.
Therefore, it had been desired to develop a polyuretahne lens having excellent releasability form a mold after cast polymerization and causing no color uneVenness during tinting and no peeling of the coating film formed thereon.
The polyurethane lenses of high refractive index produced according to the above prior art, similarly to conventional other plasitc lenses, have a big demerit of poor mar resistance. In order to improve the mar resistance, it was studied to form a cured film on these polyurethane lenses. For example, Japanese Pat. Publication Nos. 18624/1981 and 39291/1985 disclose a coatig composition composed mainly of a hydrolyzate of an organic silicon compound and colloidal silica. When the coating composition disclosed therein is applied onto a polyurethane lens, followed by the curing of the composition to form a cured film, there is obtained a polyurethane lens which is greatly improved in mar resistance and sufficiently usable. However, since this cured film has a refractive index smaller than that of the polyurethane lens base and incurs interference fringes, the cured film is not desirable viewed from the practical application.
Japanese Pat. Publication No. 48123/1986 discloses a process for producing a highly refractive lens with anti-reflection by forming on the surface of a lens made of a synthetic resin of relatively high refractive index (e.g. polymer of diallyl phthalate, diallyl isophthalate or diallyl chlorendate) a cured film of a melamine resin having a refractive index close to that of the lens base and then forming thereon an anti-reflection film made of inorganic substances. In general, when a cured film is formed from a resin of addition polymerization type such as a melamine resin, the curing temperature must be fairly high. Therefore, the cured film can have a fairly high hardness when it is formed on a substrate of good heat resistance (e.g. metallic Or glass-made substrate), but when the film is formed on a substrate of poor heat resistance e.g. plastic substrate), there is required a long curing time. In the latter case, because the curing temperature is low, the hardness of the cured film is not so high as that of the cured film formed on a metal or a glass. These problems exist also when a coating composition comprising a melamine resin is coated and cured on a polyurethane lens.
Japanese patent application Kokai (Laid-Open) No. 99236/1981 discloses a method for forming a cured film from a coating composition consisting of (a) a colloidal dispersion of at least one member selected from a metal, an alloy and a metal salt, (b) a partial condensate of an organic silicon compound represented by the general formula RSi(OH).sub.3 and (c) a catalyst for curing the partial condensate (b). The cured film obtained according to this method, having a relatively high refractive index, satisfies one of the requrements for the cured film of highly refractive plastic lens. However, since the coating composition as a starting material of the cured film compirses as the curing catalyst sodium acetate, choline acetate, etc., the practice of the above method involves such problems as the insufficient stability of the coating composition and the insufficient transparency of the cured film formed from the composition.
Japanese Pat. Publication No. 54331/1986 discloses the formation of a cured film from a coating composition consting of (a) a sol of colloidally dispersed antimony pentoxide, (b) an orgnaic silicon compound having an epoxy group and (c) at least one curing agent for the epoxy group, selected from various metal complexes and metal alkoxides. This cured film, also having a relatively high refractive index, satisfies one of the requirements for the cured film of highly refractive plastic lenses. However, when the coating composition comprises the antimony pentoxide sol in an increased amout in order to obtain a cured film of higher refractive index, the cured film tends to have cloudiness and cracks and the resulting lens is not desirable for use as a spectacle lens. This prior art uses, as the curing agent for the epoxy group, an aluminum compound such as aluminum acetylacetonate or the like. When a coating composition comprising such a curing agent for the epoxy group is coated on a polyurethane lens base and cured, the resulting cured film has insufficient adhesion to the base.
Hence, it had been desired to develop a polyurethane lens having such a cured film that has a sufficient hardness to improve the inherently low mar resistance of polyurethane lens, has excellent tranSparency and excellent adhesion to polyurethane lens base and, owing to the small difference in refractive index between the cured film and the polyurethane lens base, causes no interference fringes.